The present invention relates to an ink jet recording sheet and in particular, to an ink jet recording sheet which is extremely reduced in surface waviness of the sheet after being recorded, spread of ink dots and staining of background.
The ink jet recording method performs recording of characters and graphics by allowing ink droplets ejected by various working principles to deposit on a recording sheet such as paper. The ink jet recording has such favorable features that it makes high-speed recording possible, that is produces little noise, that it can easily perform multi-color recording, that there is no limitation as to kind of patterns or images, and that it requires no processing for development and fixing. Thus, ink jet recording is rapidly becoming widespread in various fields as devices for recording various characters including kanjis (Chinese characters) and color images. Furthermore, the images formed by the multi-color ink jet recording method are not inferior to those printed by multi-color press or those obtained by color-photography. Besides, use of the ink jet recording extends to a field of full-color image recording where number of copies is not so many, since costs per copy are less expensive than those employing the photographic process.
As for the recording sheets used for ink jet recording, efforts have been made from the aspects of printer hardware or ink composition in order to use woodfree papers or coated papers used for ordinary printing or writing. However, improvements in recording sheets have come to be required increasingly in order to go side by side with developments in printer hardware such as ever increasing speed, development of ever finer definition images of full color, and also with expanding fields of uses. That is, recording sheets are demanded to develop ever high image reproducibility, and in order to meet that demand, image density of the printed dots must be maintained high, hue characteristics must be bright and appealing, the ink applied must be fixed quickly and no bleed or spread even though a different color ink is put over additionally. Moreover, ink should set quickly, dots should not spread more than needed and the circumference of dots be sharp and demarcating.
Various proposals have been made for such demand. For example, an ink-receiving layer mainly composed of silica pigments which forms a porous layer is provided on the surface of a support to improve ink-absorbing property (Japanese Patent Application Kokai Nos. 52-9074 and 58-72495). In order to improve ink absorption property by providing an ink-receiving layer, to make printed dot density higher, and to restrain spread of ink dots, Japanese Patent Application Kokai Nos. 55-51583 and 56-157 propose to add non-colloidal silica powders to the ink-receiving layer. Based on the finding that color quality and sharpness depend on the state of the colorants distribution in the ink-receiving layer, it has been proposed to use a specific agent which adsorbs the colorant component (Japanese Patent Application Kokai No. 55-144172).
Further, as another problem encountered after recording, there is the occurrence of surface waviness of the ink jet recording sheet after printing. If the surface of the recording sheet after printing is undulating, appearance of the record appraised by senses goes down even though image reproducibility is good. The waviness is generated due to partial expansion and contraction of wood fibers in the support caused by the permeated ink. Accordingly, to inhibit permeation of the ink into the support can be a measure for avoiding the waviness. This means to provide the ink-receiving layer heavy enough to absorb most of the ink liquid, but too heavy such layer can by no means be held by binders contained in the layer and the layer will come off in a form of powders.
On the other hand, to inhibit the permeation of ink without increasing coating weight of the ink-receiving means to reduce void volume in the ink-receiving layer and the support. Reduction of void volume in the ink-receiving layer and the support causes delay in permeation of ink into the ink-receiving layer and the support, and the ink stays wet and bleeds around ink dots. Such wet ink dots stain a paper bail or other sheet transfer means of a printer, and the stain is dragged to smudge the recording sheet being printed.
In order to meet demand of recent graphics-oriented market, not only sharpness and color quality of the ink jet printed images, but also smoothness of hard copies, but also surface aesthetics is strongly demanded. The surface of sheet should be flat and smooth and free of undulation after it has received liquid ink for recording. The surface undulation or waviness, if any, also affects harmfully on runnability of the sheet on a printer since it could be a cause of jamming or smudging of the sheet. An ink jet recording sheet which inhibits ink dots spread and bleeding and is free of surface undulation after printing is demanded.
Ink jet recording sheets are classified into two categories, non-coat type and coat type. The former includes woodfree paper, bond or the like; the latter has an ink-receiving layer provided on support like paper, synthetic paper or synthetic resin film, and is further classified into light-weight coat type in which coating weight is 1-10 g/m.sup.2, medium-weight coat type in which coating weight is 10-20 g/m.sup.2, and heavy-weight coat type in which coating weight is 20 g/m.sup.2 or more. As a support for the coat type ink jet recording sheet, paper having Stockigt sizing degree of 0 to several ten seconds is commonly employed.
In the case of color recording, the amount of ink to be deposited is large, so that the coated layer alone cannot fully absorb the inks and it is necessary to use a paper support having relatively low sizing degree thereby to allow the support also to absorb a part of the deposited ink. However, if the paper support absorbs ink excessively, the ink tends to strike-through and the sheet tends to be cockling (wrinkling).
Furthermore, since ink-receiving layer is designed to absorb aqueous ink as quickly as possible, it is susceptible to humidity. That is, moisture causes difference in contraction and expansion between the support and the ink-receiving layer, resulting in curling of the sheet. Curling in turn harmfully affects runnability of the sheet on a printer.
In order to solve these problems, Japanese Patent Application Kokai No. 2-270588 proposes an ink jet recording paper having ink-receiving layers on the front and back sides of a paper support and Japanese Patent Application Kokai No. 62-282967 discloses a sheet having an ink-holding layer and ink-transfer layer on a support together with a curl inhibiting layer having properties same as or similar to those of the ink-holding layer. Furthermore, Japanese Patent Application Kokai No. 61-235184 discloses a recording material having an ink-receiving layer which is further provided with a curl inhibiting layer comprising a resin such as polyacrylamide and Japanese Patent Application Kokai Nos. 62-162586 and 62-162587 disclose that sheet transferability on a printer and blocking resistance are improved by providing a layer comprising a slight amount (0.01-1.0 g/m.sup.2) of a powder on the side of a support opposite the ink-receiving side or on both sides.
Fastness of records or images is also an important requirement for an ink jet recording sheet. The images are required to stand wetting; the ink once set should not migrate even if water is put thereon. It is further demanded that the records or images be protected from discoloring upon exposure to light, ozone gas or oxidizing gas.
In order to meet this requirement, some proposals have been made. For example, an ink jet recording paper comprising a base paper of low sizing degree which is wetted with a coating composition for surface treatment is disclosed in Japanese Patent Application Kokai No. 52-53012, and an ink jet recording sheet loaded with urea-formaldehyde resin powder and impregnated with a water-soluble polymer is disclosed in Japanese Patent Application Kokai No. 53-49113. Moreover, an ink jet recording sheet comprising a support and an ink absorbing coating layer provided on the support is disclosed in Japanese Patent Application Kokai No. 55-5830. Use of non-colloidal silica powders as pigments in the coating layer is disclosed in Japanese Patent Application Kokai Nos. 55-51583 and 56-157. Furthermore, a coated paper having two layers differing in ink absorbing rate is disclosed in Japanese Patent Application Kokai No. 55-11829.
Many examples of using cationic dye fixers for improving water resistance are disclosed, for example, in Japanese Patent Application Kokai Nos. 60-11389, 62-238783, 64-9776 and 64-77572. Many proposals have been made for improving light resistance. In addition to these proposals, many further proposals from the side of recording sheet have been made in ink jet recording method and accordingly the quality of recorded images has also been highly improved.
On the other hand, in order to ensure high image density of printed dots, sharpness and color quality, and to utilize the characteristics of dyes, it is desired that the dye of ink be fixed on the surface of the recording sheet and the vehicle of the ink alone be absorbed underneath.
As for non-coat type ink jet recording sheet, the sheet by itself is required to absorb ink and for this purpose, a non-sized paper or a slightly sized paper containing a slight amount of a sizing agent or an increased amount of a loading material can be employed. However, when recording is carried out with an aqueous ink, such recording sheet suffers from the problems that though the sheet is superior in absorbency for ink, color quality, sharpness and density of printed dots and images are low, and deterioration in the shape of dot called "feathering" (jagged in the form of bird feather) and blurring of the contour of the dot occur and the ink permeates into the base paper layer deeply and reaches the back side of the paper.
On the other hand, as for the coat type ink jet recording sheet, a recording sheet comprising a non-sized or a slightly sized paper as a support provided with a coating layer is superior in absorbing property and is improved in color quality, sharpness and inhibited from feathering of the resulting images and is inhibited from strike-through of ink as compared with the non-coated type ink jet recording sheet. Particularly, images recorded on a recording sheet provided with a coating layer containing an amorphous silica powder and a water-soluble polymer material have excellent color quality, sharpness and resolution and are inhibited from feathering and strike-through of ink.
Furthermore, as for the recording sheets having a coating layer on a support such as high-sized paper, polyethylene terephthalate film or synthetic paper which does not absorb or hardly absorbs vehicle of aqueous ink, since the support per se hardly absorbs the solvent of ink, the dye is retained on the surface of the recording sheet and image reproducibility with excellent dot density, image density, color quality, sharpness and little feathering and strike-through can be readily obtained. However, when the coating weight is small, absorbency for ink is low and particularly, ink absorbing rate in color-overlapping recording is low and absorption capacity also decreases. If the coating weight is increased for increasing the absorption capacity, the bond between the coating layer and support tends to be weak and the coated layer components tend to come off in a form of powder. Thus, improvement is needed.
These coat type ink jet recording sheets can afford high image density, excellent color quality and sharpness with less feathering and strike-through and excellent image reproducibility. It is considered that this is because voids present inside the particles such as amorphous silica powders are large and the amount of the ink absorbed in the powders, namely, in the coating layer, is large; the powders are fine powders at least 90% of which have a volume average particle size of 1-20 .mu.m; refractive index of the powders is small; and light scattering coefficient is small. However, quality demand for the ink jet recording sheet, in terms of color quality, sharpness, feathering, runnability, etc., is growing ever stringent, and improvement of the coated layer alone can no longer meet such demand.